This invention relates to the determination of certain lithological characteristics of a subsurface formation and more particularly to a method and system for testing a material sample representing the subsurface formation for its dynamic interaction characteristics with a coring fluid which might be utilized during a borehole operation for obtaining a core sample of such subsurface formation.
In the production of minerals, specifically oil and gas, it is common to "engineer" the producing reservoir to improve the economic performance thereof. To do this, certain lithological properties of the reservoir must be determined, the two most important of these properties being the permeability and the porosity of the reservoir rock. Permeability is a measure of the ability of a material to transmit fluids through pore spaces of the mineral and is inversely proportional to the flow resistance offered by the material. Porosity of a material is defined as the ratio of the aggregate volume of its void or pore spaces to its gross bulk volume. In the case of an oil reservoir, porosity is a measure of the volume within the reservoir rock which is available for storing oil and gas. Normally, porosity and permeability, as well as other chemical or physical characteristics of an earth material, are determined from core samples by applying well-defined measurement procedures.
Coring samples are ordinally taken by means of a core drill and the samples obtained are in the form of cylinders or cores. Drilling muds with a water or oil base are commonly used as coring fluids. These drilling muds are normally formulated to provide desired density and rheological properties which make them particularly suitable for use in coring wells. For example, drilling muds may be altered to increase the density by adding solid materials, such as barium sulfate, thereto. During the coring of a subsurface formation, contamination of a core sample by the drilling mud can readily occur. The core material, being porous, will be penetrated by the drilling mud filtrate under the pressure conditions present in the well. Depending on the size of the pore throats in the core material, mud solids (barite, clay minerals and rock cuttings) may also penetrate the core material. The extent of mud solids contamination of core samples must be taken into account when analyzing such core samples to identify certain subsurface formation lithological characteristics, such as porosity and permeability as examples.
In view of the foregoing, it is an object of the present invention to simulate, or model, the dynamic interaction of coring fluid on a material sample representative of a select subsurface formation, both consolidated and unconsolidated, so as to provide a measure of the extent to which a coring fluid will invade a core sample from the select formation during a conventional coring operation. This and other objects of the present invention will become apparent from the following detailed description thereof.